The present invention relates to radar processing and more particularly to a radar processor which provides enhanced detection of maneuvering targets that spread their energy over several Doppler filters.
To increase sensitivity at long ranges, or for weak targets, longer integration times may be employed. Energy is radiated over a relatively longer time in the direction of the target, and by integrating the radar return over a relatively longer time, the signal-to-noise ratio of the return signal can be improved. With the advanced processors available today, long integration times can be achieved. A combination of coherent and incoherent processing will typically be required to achieve target detection.
The received signal intensity can fluctuate as a result of target motion; such fluctuations are known as target scintillation. Target scintillation increases as the radar frequency of operation is increased. The coherent integration time is selected such that target scintillation does not reduce the sensitivity. Based on target scintillation data, coherent integration times of the order of one second are feasible.
However, for long coherent integration times, the energy of an accelerating target can spread over several
Doppler filters. The number of filters over which the energy spreads is given approximately by: EQU N=2a T.sup.2 /.lambda. (1)
where a is the acceleration, T is the coherent array time, and .lambda. is the wavelength.
Table I shows the number of Doppler filters spread as a function of coherent array time for accelerations between -0.1 g and +0.1 g. As can be seen even for low accelerations, large spreadings may result.
TABLE I ______________________________________ Target Spreading Due to Acceleration (-0.1 g .ltoreq. a .ltoreq. +0.1 g and .lambda. = .14 ft.) T (sec) N (number of filters) ______________________________________ .22 .+-.2.25 .44 .+-.9.00 .66 .+-.20.25 ______________________________________
The signal-to-noise loss in the center filter as a function of the number of filters spread is shown in FIG. 1. The loss is with respect to a non-accelerating target and uniform amplitude weighting is assumed.
Large target accelerations and/or long coherent arrays can produce significant losses in target signal-to-noise ratio and degrade detection. One method which has heretofore been considered to compensate for accelerating targets is to apply a quadratic phase shift .phi. to the data before the Fast Fourier transform (FFT): EQU .phi.=2.pi.(a/.lambda.)(n/PRF).sup.2
where n is the sample number, a is the acceleration and PRF is the radar pulse repetition frequency.
A disadvantage of this method is that a separate FFT operation is required for each acceleration. This technique is impractical for large accelerations or long integration times because of high processor loading.
It would therefore represent an advance in the art to provide a radar processor which performs target acceleration compensation without high processor loading.